from the GNU version of the Collaborative International Dictionary of English

prep. Through; by means of; through the agency of; by; for; for each. Per is also sometimes used with English words.

from The Century Dictionary and Cyclopedia

Through; by means of.

A prefix of Latin origin, meaning primarily ‘through.’ See the etymology.

As an inseparable prefix of intensity, ‘thoroughly,’ ‘very,’ as in peracute, perfervid, pellucid; specifically, in chem., noting the maximum or an unusual amount, as peroxid, the highest oxid, or an oxid containing more oxygen than the protoxid, etc.

n. In petrography, in the quantitative system of classification of igneous rocks (see rock), a prefix used to form adjectives, and showing that the factor or component indicated is present in any division of igneous rocks, alone or in extreme amount — that is, that its ratio to another factor is greater than : as, peralkalic, perfelic, perfemane, perfemic, etc.

n. An abbreviation of period.

Etymologies

Latin; see per1 in Indo-European roots.

(American Heritage® Dictionary of the English Language, Fourth Edition)

From Latin per ("through, during"), from Proto-Indo-European *per. (Wiktionary)

shortening of person, coined by Marge Piercy in Woman on the Edge of Time (1979) (Wiktionary)

Examples

Kilwardby also stresses that in a per se necessity sentence, the subject must be ˜something belonging in itself to that predicate™ (˜per se aliquod ipsius predicati™), by which he seems to mean that the subject has the predicate as an essential property, i.e., such that it has the predicate as a necessary property through itself and not through something else.

I've seen that "5% success rate" in discovery during several nasty disputes--I'm a hired gun called in AFTER the execrement has hit the rotary air-moving device--and every time further inquiry revealed that the percentage rate was per _submission event_, not _per manuscript_.

A limit of sixteen months is assigned, within which pledges must be redeemed or they become the property of the pawnbroker; and the interest charged, formerly four per cent., is now fixed at three per cent. _per month_.

Peruvian guano, sulphate of ammonia, soda-saltpeter, fish and flesh manures, bones and urine, cost the farmer more money per ton than any other manures he buys or makes, superphosphate of lime excepted, and this does not find sale, for general purposes, unless it contains several _per cent. _ of nitrogen.

The witnesses had all been placed on an allowance of thirty francs per diem, which seems as good a device to invalidate their evidence as could have been adopted, and many are supposed to have come forward only _per chiappar il denaro_.